No. In fact, ActivityPub has no general mechanism for even knowing where content has been distributed to. So when you ask your instance to delete something, it can't actually know what other instances to ask to delete the mirrored content.

Mastodon tries its best by sending deletion requests to all known instances, in the hope that that will reach all instances that have fetched the content. But in fact, instances that are unknown to your own instance could have the content as well, though this is probably a very rare occurrence.

Bottom line: Don't write anything on the internet that you don't want publicly displayed. Anyone can save it and then you can't force them to delete it. That applies to the entire internet. It also applies to the fediverse.

There is no legal precedent, but most likely you would only have to prove deletion on your own server.